Gas-phase synthesis method for forming semiconductor nanowires

ABSTRACT

The present invention provides a method and a system for forming wires ( 1 ) that enables a large scale process combined with a high structural complexity and material quality comparable to wires formed using substrate-based synthesis. The wires ( 1 ) are grown from catalytic seed particles ( 2 ) suspended in a gas within a reactor. Due to a modular approach wires ( 1 ) of different configuration can be formed in a continuous process. In-situ analysis to monitor and/or to sort particles and/or wires formed enables efficient process control.

CROSS-REFERENCE TO OTHER APPLICATIONS

This application is a national phase application under 35 U.S.C. §371 ofinternational application PCT/SE2011/050599 (filed May 11, 2011) whichclaims priority to Swedish Application 1050466-0 (filed May 11, 2010).

TECHNICAL FIELD OF THE INVENTION

The present invention relates to formation of wires and in particular togas-phase synthesis of wires in the absence of a substrate. Thegas-phase synthesis is applicable to different materials, and inparticular to semiconductor materials.

BACKGROUND OF THE INVENTION

Small elongated objects, usually referred to as nanowires, nanorods,nanowhiskers, etc. and typically comprising semiconductor materials,have up till now been synthesized using one of the following routes:

-   -   liquid phase synthesis, for example by means of colloidal        chemistry as exemplified US 2005/0054004 by Alivisatos et al,    -   epitaxial growth from substrates, with or without catalytic        particles as exemplified by the work of Samuelson et al        presented in WO 2004/004927 A2 and WO 2007/10781 A1,        respectively, or    -   gas phase synthesis by means of a laser assisted catalytic        growth process as exemplified by WO 2004/038767 A2 by Lieber et        al.

The properties of wires obtained using these routes are compared in thefollowing table.

Width/ length Scalability/ Material and size Structural cost of qualitycontrol complexity production Liquid HIGH THIN/ LOW HIGH/HIGH phaseSHORT MEDIUM control Substrate- HIGH ALL/ALL HIGH LOW/HIGH based HIGHcontrol Laser MEDIUM THIN/ LOW MEDIUM/ assisted LONG MEDIUM/ MEDIUMcontrol

Consequently, the choice of synthesis route is a compromise betweendifferent wire properties and cost of production. For examplesubstrate-based synthesis provides advantageous wire properties butsince wires are formed in batches the scalability of the process, andthus the production cost and through-put, are limited.

SUMMARY OF THE INVENTION

In view of the foregoing one object of the invention is to provide amethod and a system for forming wires that enables a large scale processcombined with a structural complexity and material quality comparable towires formed using substrate-based synthesis.

The method comprises the basic steps of

providing catalytic seed particles suspended in a gas,

providing gaseous precursors that comprises constituents of the wires tobe formed,

passing the gas-particle-precursor mixture through a reactor, typicallya tube furnace, and

growing the wires from the catalytic seed particles in a gas-phasesynthesis including the gaseous precursors while the catalytic seedparticles are suspended in the gas.

In a first aspect of the invention wires of different configuration suchas wires made of essentially the same material, unipolar wires, or morecomplex wires such as wires with axial pn- or pin-junctions, wires withradial pn- or pin-junctions, heterostructure wires, etc. can be providedby varying the growth conditions during growth of each wire, such that awire segment is axially grown on a previously formed wire portion in alongitudinal direction thereof, or a shell is radially grown on thepreviously formed wire portion in a radial direction thereof, ormaterial is added as a combination of axial and radial growth. Thegrowth conditions can be varied between the reaction zones bycontrolling one or more of parameters associated with: precursorcomposition, precursor molar flow, carrier gas flow, temperature,pressure or dopants. This variation is in practice achieved byperforming the wire growth in two or more zones, which may be kept atdifferent temperature, and into which suitable growth or dopantprecursor molecules are injected by means of mass flow controllers orsimilar devices.

Growth conditions can also be varied over time by controlling one ormore of parameters associated with: precursor composition, precursormolar flow, carrier gas flow, temperature, pressure or dopants, or thesize distribution of the catalytic seed particles, such that the wireproperties can be varied from time to time, either to produce a batchwith a range of different wires, or to produce distinct homogeneousbatches.

The catalytic seed particles can be provided as an aerosol that is mixedwith the gaseous precursors prior to, or during, initiation of wiregrowth. Alternatively the catalytic seed particles are formed byformation from gaseous reactants that comprises at least one of theconstituents of the catalytic particles, thereby enabling aself-catalyzed wire growth.

Preferably, the method of the invention comprises providing a flow ofthe gas that carries the catalytic seed particles and subsequently thepartly or fully formed wires through one or more reactors, each reactorcomprising one or more reaction zones. Thereby the catalytic seedparticles and any wires formed thereon flow sequentially through one ormore reaction zones, where each reaction zone contributes to the wiregrowth by adding material to the wire or etching the wire. This enablesto provide optimum conditions for each step in the growth process.

The diameter of the wires is partly determined by the size of thecatalytic particles. Thus the diameter of the wires can be controlled bychoosing an appropriate size or size distribution of the catalytic seedparticles and by adjusting the growth conditions to the size of thecatalytic seed particles.

In the case of a second reaction furnace or reaction zone, continuedwire growth occurs on pre-fabricated semiconductor wires with attachedcatalytic particles, formed in the first reactor. These wires act asflying substrates, and consequently growth will take place more readilythan in the first zone, where wire nucleation takes place on the seedparticles. Therefore, wire growth in subsequent furnaces is moreefficient and takes place at lower temperatures. Depending on growthconditions (reactor temperature and pressure, precursor type andconcentration, seed particle/wire size and concentration, and reactiontime) the subsequent wire growth takes place in the axial or radialdirection, or as a combination of both.

In one aspect of the invention, the method comprises addition of HCl orother etching halide compound to the flow of aerosol, to emulate theconditions in hydride vapour phase epitaxy, HVPE, preventing growth onthe hot wall of the reactor. HVPE sources, where metallic group-IIIatoms are carried as chlorides to the reaction zone, can also be used inthis invention.

In another aspect of the invention, the seed particles/wires are heatedby means of microwaves, infrared light or other electromagneticradiation, instead of or as a complement to the hot wall tube furnace.This allows the gas to remain more or less cold, minimizing the amountof gas-phase reactions, while allowing growth on the hot particle/wiresurfaces.

In yet another aspect of the invention the method comprises in-situanalysis of the wires or the partly grown wires to obtain the desiredwire properties. Means for controlling the wire growth involve controlof the size of the catalytic seed particles, but also control of growthconditions by controlling one or more of parameters associated with:precursor composition, precursor molar flow, carrier gas flow,temperature, pressure or dopants, in one or more of the reaction zonesmentioned above. The in-situ analysis provides means for obtainingfeed-back in a control loop not available in for example substrate-basedsynthesis. Any deviation from desired properties is rapidly detected andthe growth conditions can be adjusted without significant delay orwithout having to discard a significant number of wires.

Means for in-situ analysis include means for detecting the size of thecatalytic seed particles and/or the wires formed, such as a differentialmobility analyser (DMA), illumination and detection of luminescence fromthe wires formed, absorption spectroscopy, Raman spectroscopy and X-raypowder diffraction on-the-fly, etc. In addition to the possibility tocontrol the wire growth in “real-time” the in-situ analysis can also beused to selectively sort wires having different properties, such assize. Although described in terms of wires, it should be appreciatedthat the in-situ analysis can be performed also on catalytic seedparticles, or partly formed wires.

In yet another aspect of the invention the method comprises collectionof the wires from the gas that carries the wires. The wires can becollected and stored for later use or they can be transferred to adifferent carrier or a substrate to be incorporated in some structure toform a device.

To take advantage of the continuous flow of wires the wires may bedeposited and/or aligned on a substrate in a continuous process, such asa roll-to-roll process. The deposition and/or alignment can be assistedby an electric field applied over the substrate and further by chargingthe wires, and optionally also the substrate. By local charging of thesubstrate in a predetermined pattern wires can be deposited inpredetermined positions on the substrate. Thus the present inventionprovides a continuous, high through-put, process for manufacturingaligned wires on a substrate, optionally with “real-time” feed-backcontrol to obtain high quality wires.

The wires produced by the method of the invention can be utilised torealise wire based semiconductor devices such as solar cells, fieldeffect transistors, light emitting diodes, thermoelectric elements,field emission devices, nano-electrodes for life sciences, etc which inmany cases outperform conventional devices based on planar technology.

Although not limited to nanowires, semiconductor nanowires produced bythe method of the invention possess some advantages with respect toconventional planar processing. While there are certain limitations insemiconductor devices fabricated using planar technology, such aslattice mismatch between successive layers, nanowire formation inaccordance with the invention provides greater flexibility in selectionof semiconductor materials in successive segments or shells and hencegreater possibility to tailor the band structure of the nanowire.Nanowires potentially also have a lower defect density than planarlayers and by replacing at least portions of planar layers insemiconductor devices with nanowires, limitations with regards todefects can be diminished. Further, nanowires provide surfaces with lowdefect densities as templates for further epitaxial growth. As comparedto substrate-based synthesis lattice mismatch between substrate and wiredoes not have to be considered.

The apparatus of the invention comprises at least one reactor forgrowing wires, said reactor comprising one or more reaction zones, meansfor providing catalytic seed particles suspended in a gas to thereactor, means for providing gaseous precursors that comprisesconstituents of the wires to be formed to the reactor, and means forcollecting wires grown from the catalytic seed particles in a gas-phasesynthesis including the gaseous precursors while the catalytic seedparticles are suspended in the gas.

A plurality of reactors, each providing a reaction zone, or reactorsthat are divided into different reaction zones, or a combination thereofcan be used to enable change of growth conditions during growth of eachwire. During processing the catalytic particles, the partly grown wiresand the fully grown wires are carried by a gas flow sequentially throughthe reactors.

Preferably the apparatus further comprises means for in-situ analysis ofthe wires formed. In one embodiment of the invention said means forin-situ analysis is arranged for detection of wire properties after oneof said reaction zones and a signal from said means for in-situ analysisis fed back to a means for controlling the growth conditions upstream.

One advantage of the method and apparatus in accordance with theinvention is that wires can be grown at a surprisingly high rate. Growthrates may be higher than 1 μm/s, which implies a growth time of a fewseconds for a typical wire of 0.4×3 μm dimension. This means that, in acontinuous process in accordance with the invention the through-put istremendous.

Embodiments of the invention are defined in the dependent claims. Otherobjects, advantages and novel features of the invention will becomeapparent from the following detailed description of the invention whenconsidered in conjunction with the accompanying drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will now be described withreference to the accompanying drawings, wherein

FIG. 1 schematically illustrates axial growth of a nanowire inaccordance with the invention,

FIG. 2 schematically illustrates a system for forming wires, in (a) witha single reactor, and in (b) extended to a modular system with aplurality of reactors, in (c-h) examples of different sub-modules inaccordance with the invention,

FIG. 3 schematically illustrates axial growth of a wire comprising apn-junction in accordance with the invention,

FIG. 4 schematically illustrates core-shell growth of a wire comprisinga pn-junction in accordance with the invention,

FIG. 5 schematically illustrates a system for forming wires comprisingin-situ analysis modules in accordance with the invention,

FIG. 6 schematically illustrates a first embodiment of a system forforming nitride based LED structures with different emission wavelengthsin accordance with the invention,

FIG. 7 schematically illustrates a second embodiment of a system forforming nitride based LED structures with different emission wavelengthsin accordance with the invention,

FIG. 8 schematically illustrates an arrangement for in-situ photoluminescence measurements in a system for forming wires in accordancewith the invention,

FIG. 9 schematically illustrates an arrangement for in-situ absorptionmeasurements in a system for forming wires in accordance with theinvention, and

FIG. 10 and FIG. 11 shows wires of different configuration formed in asystem in accordance with the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

For the purpose of this application the term wire refers to an elongatedobject. As mentioned above, these wires may be of essentially nanometerdimensions in their width or diameter, commonly referred to asnanowires, nanowhiskers, nanorods, etc., however not limited to this.

Referring to FIG. 1, basically a method for forming wires in accordancewith the invention comprises

-   -   providing catalytic seed particles 2 suspended in a gas,    -   providing gaseous precursors 3, 4 that comprises constituents of        the wires 1 to be formed, and    -   growing the wires 1 from the catalytic seed particles 2 in a        gas-phase synthesis including the gaseous precursors 3, 4 while        the catalytic seed particles are suspended in the gas.

The growth, or at least part thereof, is performed at an elevatedtemperature, typically in a furnace or some other kind of reactor, andstarts with an initiation of the growth by catalytic decomposition ofthe gaseous precursors 3, 4 on the surface of the catalytic seedparticles 2 and nucleation. After nucleation the wire 1 growsdirectionally and forms an elongated object, i.e. a wire. Preferably thegas flows through the reactor and thereby carries at least the catalyticseed particles and thus the wires formed on the catalytic seed particlesthrough the reactor.

The method is described herein in terms of semiconductor materials, inparticular III/V-materials, however not limited to this. By way ofexample, FIG. 1 schematically illustrates the formation of a GaAs wire 1from a catalytic seed particle 2, such as gold, and gaseous precursorsTMGa 3 and AsH₃ 4. As shown the catalytic particles are carried forwardby the gas into a reactor where the gaseous precursors 3, 4 are presentand the reaction takes place. The precursor gases may be added to thegas flow prior to entering the reactor or directly to the reactor.

A basic system for forming wires in accordance with the presentinvention is schematically illustrated in FIG. 2a . The system comprisesat least one reactor 8 for growing wires 1, means 9 for providingcatalytic seed particles suspended in a gas to the reactor 8, and means10 for providing gaseous precursors 3, 4 that comprises constituents ofthe wires 1 to be grown from the catalytic seed particles in a gas-phasesynthesis including the gaseous precursors while the catalytic seedparticles are suspended in the gas. Optionally the system furthercomprises means 11 for collecting the wires 1. The system may alsocomprise means for in-situ analysis 12 of particles and wires formed inthe reactor 8, such as differential mobility analyzers (DMAs) or otheranalysis tools to monitor the dimension or other properties of thewires.

In one embodiment of the method of the invention the wire growth isperformed in one or more reactors arranged in sequence and/or inparallel, where a continuous flow of catalytic seed particles issupplied as an aerosol, which is mixed with gaseous precursors 3, 4, andthen the gas mixture enters a first reactor of said one or more reactorswhere the wire growth is initiated. The catalytic seed particles 2 canalso be formed by gaseous reactants inside said first reactor, therebyenabling a self-catalyzed wire growth. When performing the wire growthin a plurality of reactors, each reactor increases the complexity of thewires, e.g., to make pn-junctions or heterostructures in the axial orradial direction.

The reactors, the means for providing catalytic seed particles, meansfor in-situ analysis, etc. of said system do not have to be separatechambers or arrangements. Preferably the system is a modular systemcombined in an in-line production apparatus. In particular, each reactormay comprise one or more reaction zones arranged in sequence and/or inparallel as described for the reactors above. Hence since a reactionzone has the same function as a reactor, these terms are interchangeablyused hereinafter. FIG. 2b schematically illustrates such a modularsystem with particle delivery system 9, several growth modules arrangedin series and in parallel and means for collecting the particles andwires being carried out from the growth modules by the gas flow. FIG. 2shows other examples of modules that can be incorporated in the system:(c) a wire growth module, (d) a shell growth module, (e) a passivationlayer growth module, (f) an in-situ analysis tool 12 (with the arrowindicating the possibility to feed-back control), such as a DMA, (g) anevaporation module with an evaporation source 13 and (h) aplasma-enhanced chemical vapour deposition module with a plasma source14, however not limited to this.

FIG. 3 schematically illustrates how the method of the invention can beused to form a GaAs wire comprising an axial pn-junction between ap-doped GaAs segment and an n-doped GaAs segment. Precursors 3, 4comprising group III material and group V material, respectively, andp-dopants are provided to a reactor and, after nucleation, p-doped GaAsis axially grown from the catalytic seed particle, thereby forming afirst axial segment of the GaAs wire. Thereafter the growth conditionsare changed by exchanging the p-dopant to an n-dopant, whilesubstantially maintaining other parameters related to the growthconditions, such that a second axial wire segment is axially grown onthe previously formed first segment in a longitudinal direction thereof.This illustrates the possibility to vary the growth conditions duringaxial growth to obtain axial segments with different properties.

FIG. 4 schematically illustrates the formation of a GaAs wire comprisinga radial pn-junction between a p-doped GaAs core and an n-doped GaAsshell. Precursors 3, 4 comprising group III material and group Vmaterial, respectively, and p-dopants are provided to the reactor andafter nucleation p-doped GaAs is axially grown from the catalytic seedparticle, thereby forming the core of the GaAs wire. Thereafter thegrowth conditions are changed by increasing the temperature and/or theV/III-ratio to promote radial growth and by exchanging the p-dopant toan n-dopant. Thereby the shell is radially grown on the previouslyformed core in a radial direction thereof. This illustrates thepossibility to vary the growth conditions to switch between axial growthand radial growth.

Although exemplified with GaAs, it should be appreciated that otherIII/V semiconductor materials as well as semiconductor materialscomprising group II and group VI materials can be processed in the sameway. For example the gaseous precursors of the above examples can beexchanged for TMIn and PH₃ to form InP wires. As appreciated to a personskilled in the art the reactor configuration does not have to be changedto form wires from different gaseous precursors, the gaseous precursorsare simply switched. Moreover, the processes such as those exemplifiedby FIG. 3 and FIG. 4 can be performed with or without dopant. Insulatorscan also be grown. Single or multiple reactors or reaction zones withina reactor can be used to improve formation of segments, cores or shellshaving different composition, doping or conductivity type. Moreover,axial and radial growth is not necessarily fully decoupled but the wirecan grow both radially and axially at the same time. By choosingappropriate gaseous precursor, flows, temperatures, pressures, andparticle sizes, the wire material can be made to grow in the axial orradial direction, or in a combination of the two growth modes.

The catalytic seed particles may consist of a single element, or acombination of two or more elements, to assist in the wire growth ordope the wire. Gaseous precursors may also be used to dope the wire.

In case of pre-forming the catalytic seed particles said means forproviding catalytic seed particles 9 may comprise a particle generator.The particle generator produces an aerosol of more or less size-selectedparticles by a range of prior art methods. Particle generation can bedone by evaporation/condensation, spray or vapor pyrolysis, sparkdischarge, laser ablation, electrospraying of colloidal particles, etc.Size selection can be done by gas mobility classification, e.g. by usinga DMA, virtual impaction, or simply well-controlled particle formation.For many applications, it is desirable that the aerosol particles beelectrically charged, which can be accomplished by radioactive sources,corona discharge, thermal or optical emission of electrons, etc. Atypical system for particle generation is described in Magnusson et al.,Gold nanoparticles: production, reshaping, and thermal charging, JNanoparticle Res 1, 243-251 (1999).

As mentioned above, the system may comprise one or more reactors orreaction zones, where each reactor or reaction zone adds a newfunctional layer to the wires. Such a modular system is shown in FIG. 5,and is further described below. Depending on the growth parameters, suchas precursor molecules, temperature, pressure, flows, particle densityand particle size, the new functional layer can be added as an axialextension of the previously formed wires, as a radial shell, or as acombination of both axial and radial growth. The formed layers can be ofsimilar or dissimilar material, i.e., homo- or heteroepitaxy, and ofsimilar or dissimilar conduction type, e.g., a pn-junction. Thefunctional layers are not limited to crystalline layers formed byepitaxy but can also be amorphous layers such as oxides, providing apassivating and/or insulating functionality. Chemical reactions to coatthe wires with surfactants or a polymer shell, or condensation ofsacrificial layers for later re-dispersion are other possibilities.

For some growth conditions, additional modules may be added to thereactor or the reaction zone. For example a plasma generator may beadded to modify the chemical reactions to enable higher reaction rates.This is important especially if the wire or layer formed on the wire isgrown at low temperature by a stable precursor which usually requires ahigh temperature to decompose. A typical example where this may beuseful is for growth of nitrides from ammonia.

Before or between the reactors or reaction zones, further components maybe placed, for example means for charging particles or wires. Atube-shaped absorption filter can be used to remove precursor moleculesand small particles from the gas flow, by taking advantage of acomparatively low diffusion coefficient of the wires. Precursors andreactants can thereby be replaced, not only added, between the growthreactors. Size classification tools, such as DMA or virtual impactor,can also be used to refine the gas flow, i.e. the aerosol, or as in-situanalysis as explained below.

Referring to FIG. 5, in the following one implementation of the methodof the invention is described in terms of growing GaAs wires containinga pn-junction. The system comprises a particle delivery system that canconsist of any of the prior mentioned particle generators. The particlesare generated and then carried by a gas flow such as H₂ or N₂ from theparticle delivery system. Hereinafter, the gas flow containing particlesor wires is termed an aerosol. The GaAs (n-type) wire growth moduleconsists of a reaction furnace and a gas delivery system for theprecursor molecules. In this case the precursor molecules are TMGa, AsH₃and SiH₄. TMGa and AsH₃ form the GaAs material while SiH₄ dopes thewires with Si resulting in an n-type material. The precursor moleculesare mixed with the aerosol prior to entering the reaction furnace. Uponentering the reaction furnace the precursors react with the particles inthe aerosol forming n-type GaAs wires. The growth parameters(temperature, flows, pressure etc.) are modified to obtain the desiredproperties (length, crystal structure, shape etc.). After the GaAs(n-type) wire growth module, the aerosol, which now consists of thecarrier gas and n-type GaAs wires, exits the GaAs wire growth module,and is divided into a small flow and a large flow. The small flow entersa DMA which analyzes the wires size distribution. The larger flow entersthe next wire growth module. The GaAs (p-type) wire growth module isdesigned to grow an axial extension of p-type GaAs on top of thepreviously grown n-type GaAs wires. The growth module has essentiallythe same design as the GaAs (n-type) wire growth module except for theprecursors which now consist of TMGa, AsH₃ and DEZn. TMGa and AsH₃ formthe axial extension of GaAs material while DEZn dopes the wires with Znresulting in a p-type material. The growth parameters in this furnaceare not necessarily the same as in the previous growth module but areinstead optimized to obtain an axial extension of the wires with a highquality p-type GaAs material. Upon exiting the GaAs (p-type) wire growthmodule the aerosol is divided in a small and a large flow. The smallflow enters a DMA which analyzes the wires size distribution. The largeflow enters a wire collection module which can collect the wires by anyof the prior mentioned methods.

By using a plurality means for in-situ analysis, such as the two in-situDMAs of FIG. 5, the wire growth process can be monitored at intermediatestates of the wire growth and if necessary, growth parameters can beadjusted to obtain consistent, high quality wires with the desiredproperties.

As mentioned above, said method and system of the invention can be usedto form complex wire structures. By way of example, FIG. 6 schematicallyillustrates a system for growth of nitride-based light emitting diodes(LEDs) adapted to provide emission at different wavelengths. The systemcomprises a particle delivery system a GaN (n-type) wire growth modulearranged in series followed by InGaN shell growth modules arranged inparallel prior to an AlGaN (p-type) shell module and finally a means forparticle/wire collection. Hence, the gas flow is divided into parallelInGaN shell growth modules that are adapted to form InGaN shells havingdifferent composition, i.e. In_(x)Ga_(1-x)N, In_(y)Ga_(1-y)N andIn_(y)Ga_(1-y)N where x≠y≠z. Due to the different growth conditions ineach of the branches the wires will obtain different emissioncharacteristics. For example, wires adapted for emission in the red,green and blue wavelength regions can be accomplished. By collecting theat least partly formed wires from the InGaN shell growth modules into acommon gas flow the different wires can be grown and collectedsimultaneously for assembly of white light LEDs.

FIG. 7 schematically illustrates a similar system as shown in FIG. 6,although with the possibility of more control during growth sincedifferent InGaN quantum wells get different shells individually adaptedfor the quantum well structure. In addition to the parallel InGaN shellgrowth modules of the system of FIG. 6, each InGaN shell growth moduleis followed by a p-AlGaN growth module. However, a n-GaN wire growthmodule and a AlO passivation layer growth module following the p-AlGaNshell growth modules may be the same for different wires in order toreduce the complexity of the system.

The flexibility of the system allows for several in-situ analysis tools12, to measure and monitor properties which are not obtainable usingother wire growth techniques. This allows instant feedback to regulatethe system, making it possible to continuously fine-tune materialparameters in a way that is not possible in other methods.

By way of example, wire size measurement and sorting is achievable byusing a DMA. The DMA, or any other means for in-situ analysis, can becoupled either in series or in parallel, depending on if the measurementis to be invasive or non-invasive on the gas flow. Coupled in series aDMA can sort the wires in the aerosol by their size. The size and sizedistribution which is sorted depends on the properties and settings ofthe DMA. Coupled in parallel, a small aerosol flow can be extracted tothe DMA for an almost non-invasive measurement. In this case the DMA canscan within its size detection range to give the size distribution ofthe aerosol. This can be done while only wasting a small part of the gasflow thus maintaining a high production rate of wires.

By illuminating the gas flow, the optical properties of the wires can bestudied in a non-invasive manner. The light source should preferably bea laser where the energy of the light is higher than the band gap of oneor more materials that the wires consist of. By using a photodetector,the luminescence from the wires can be studied. This enables monitoringof the optical properties of the wires, which can be used to tune growthparameters to obtain the desired properties of the wires. This is incontrast to other growth methods in that the wires may be cooled downrapidly after each successive growth reactor or reaction zone and thetemperature sensitive photoluminescence technique can be used betweeneach step in the wire growth.

Further possible in-situ optical methods include absorptionspectroscopy, where the absorption path would ideally be along the wireflow; Raman spectroscopy (especially Coherent anti-Stokes RamanSpectroscopy, CARS), which can also be used inside reaction furnaces tostudy decomposition of molecules and temperature gradients; and X-raypowder diffraction on-the-fly.

Depending on the type of wires being produced, different collectionmethods are possible. For charged wires, they are easily collected onany substrate by means of an electric field. The aerosol may be bubbledthrough a liquid to remove the wires from the gas flow, with or withoutsurfactant molecules to keep the wires from agglomerating. Wires thatare easily re-dispersed may be collected in a filter as a dry powder.

FIG. 8 schematically illustrates an arrangement for in-situ photoluminescence (PL) measurements in a system for forming wires inaccordance with the invention. This PL arrangement comprises a lightsource and a photodetector arranged at e.g. a transparent quartz tube.For an appropriate luminescence measurement the light source should be alaser with light of higher energy than the bandgap of the semiconductormaterial of the wires flowing through said transparent quartz tube.

FIG. 9 schematically illustrates an arrangement for in-situ absorptionmeasurements in a system for forming wires in accordance with theinvention. This in-situ absorption measurement arrangement comprises alight source and an absorption detector arranged at e.g. a transparentquartz tube. For an absorption measurement the light should emanate froma white light source with collimated light. The absorption detector ispreferentially placed in this alignment to the light source in order tomaximize the absorption volume of the aerosol.

As a further example of wires formed by the method and the system of theinvention FIG. 10 and FIG. 11 shows scanning electron microscope (SEM)images of GaAs nanowires grown under two different growth conditions,hereinafter referred to as (i) and (ii), respectively. Au agglomeratesare generated from molten Au in a high temperature furnace with a settemperature of (i) 1775° C. or (ii) 1825° C. The Au agglomerates arecarried by 1680 sccm of N₂ carrier gas (hereinafter the carrier gascontaining Au agglomerates/particles is termed aerosol) between thedifferent modules of the growth system. After the high temperaturefurnace the Au agglomerates are charged with a single electron each. Byusing this single electron charge the Au agglomerates are size selectedby a differential mobility analyzer, in this case set at 50 nm. Theaerosol is passed through a sinter furnace with a temperature of 450° C.which compacts the Au agglomerates into spherical Au particles. Afterthe sinter furnace the aerosol is mixed with the precursor gases TMGaand AsH₃, with a set molar flow of 2.4*10⁻² mmol/min and 2.2*10⁻²mmol/min respectively. The aerosol, including the precursor gases,enters the reaction furnace set to a temperature of (i) 450° C. or (ii)625° C. Inside the reaction furnace the precursors decompose to form thematerial constituents Ga and As. The material constituents are suppliedto the Au particles in the gas phase and a GaAs seed crystal isnucleated on the Au particle. The continued growth of the wire proceedsvia two different growth modes, (i) an axial growth mode where materialis incorporated in the interface between the Au particle and the GaAsseed crystal forming a wire, (ii) a combination of an axial and radialgrowth mode where material constituents are incorporated both at the Auparticle-GaAs interface and on the side facets of the wire that isformed, forming a wire with a conical shape. After the reaction furnacethe wires are transported by the carrier gas to a deposition chamberwhere a voltage of 6 kv is applied to a Si substrate to deposit theelectrically charged wires. As shown in FIG. 10 the Au particle isvisible and has a bright contrast compared to the darker nanowires. Asshown in FIG. 11 the Au particle is visible having a bright contrast atthe tip of the conically shaped nanowire.

The formation of GaAs nanowires typically takes place in the temperatureregime between 380° C. and 700° C. depending on the desired shape andproperties of the formed nanowires. A higher temperature typicallyresults in a higher growth rate, i.e., longer nanowires for a set growthtime, but also in a conical shape, along with effects on crystalstructure and impurity incorporation. Besides temperature, the ratio ofgroup V material precursor to group III material precursor, i.e., theV/III ratio, is important. If the V/III ratio is too low, typicallybelow 0.2, the nanowire growth proceeds in a group III rich environmentwhich can reduce the growth rate and material quality. If the V/IIIratio is too high, typically above 5, the nanowires are difficult tonucleate since group III material can't be dissolved in the Auparticles. Formation of GaAs nanowires typically takes place with atotal pressure inside the reactor between 50 and 1100 mbar. A lowerpressure reduces the supersaturation in the gas phase which can reduceparasitic gas phase reactions. A higher pressure increases thesupersaturation in the gas phase which can increase the supersaturationin the Au particle and increase the growth rate. The pressure can alsobe used to control the residence time in the growth reactor.

It should be noted that parameters such as temperature, precursor flow,V/III ratio and pressure are dependent on the precursor molecules thatare used since only the material that actually reaches the growthinterface is incorporated. If a precursor can withstand highertemperatures without reacting, the nanowire-forming reaction most likelytakes place at a higher temperature.

The above discussion on growth parameters is valid mainly for singlestage growth, where nucleation and wire growth take place in a singlereaction zone. For multiple stage growth, the first nucleation stageshould typically be done at a higher temperature, lower precursor flowand lower V/III ratio, compared to the subsequent growth steps.

Compared with MOVPE nanowire formation in the described processtypically takes place at a lower V/III ratio but at similartemperatures. Since parameters such as temperature, pressure, flows andV/III ratio are dependent on the exact chemistry used to form thenanowires it is understood that different materials may be formed atdifferent parameters. For example III-nitrides may be formed at highertemperatures due to the higher stability of the NH₃ precursor, whileInAs growth is done at lower temperatures.

Suitable materials for formation of the wires of the method and thesystem in accordance with the invention include, but are not limited to:

-   -   InAs, InP, GaAs, GaP and alloys thereof        (In_(x)Ga_(1-x)As_(y)P_(1-y))    -   InSb, GaSb and alloys thereof (In_(x)Ga_(1-x)Sb)    -   AlP, AlAs, AlSb and alloys thereof for example AlP_(1-x)As_(x).    -   InGaAsP alloyed with Al, for example Al_(x)Ga_(1-x)As    -   InGaAsP alloyed with Sb, for example GaAs_(y)Sb_(1-y)    -   InN, GaN, AlN and alloys thereof (In_(x)Ga_(1-x)N)    -   Si, Ge and alloys thereof, i.e. (Si_(x)Ge_(1-x))    -   CdSe, CdS, CdTe, ZnO, ZnS, ZnSe, ZnTe, MgSe, MgTe and alloys        thereof    -   SiO_(x), C (Diamond), C (Carbon nanotube) SiC, BN

Suitable materials for the catalytic seed particle include, but are notlimited to:

-   -   Au, Cu, Ag    -   In, Ga, Al    -   Fe, Ni, Pd, Pt    -   Sn, Si, Ge, Zn, Cd    -   Alloys of the above, e.g., Au—In, Au—Ga, Au—Si

Suitable gases for carrying the catalytic seed particles and the wiresin the process include, but are not limited to: H₂, N₂ or a mixturethereof; or He, Ar.

Suitable dopants include, but are mot limited to, for

-   -   InGaAl—AsPSb system: n-dopants: S, Se, Si, C, Sn; p-dopants: Zn,        Si, C, Be    -   AlInGaN system: n-dopants: Si; p-dopants: Mg    -   Si: n-dopants: P, As, Sb; p-dopants: B, Al, Ga, In    -   CdZn—OSSeTe system: p-dopants: Li, Na, K, N, P, As; n-dopants:        Al, Ga, In, Cl, I

According to common nomenclature regarding chemical formula, a compoundconsisting of an element A and an element B is commonly denoted AB,which should be interpreted as A_(x)B_(1-x)

It should be appreciated that the wire growth may comprise one or moreetch steps, where material is removed rather than grown on the wires.Etching can also be used to decouple radial and axial growth, which forexample enables lowering of the tapering of the wires or simple shapecontrol of the wires.

The size of the wires depends on many factors such as the materialsforming the wires, the intended application for the wires and therequirement on quality of the wires formed. Preferably the wires havediameter of less than 10 μm, and more preferably, in particular forformation of wires comprising lattice mismatched layers or segments, thewire diameter is less than 300 nm.

Since the wires of the invention may have various cross-sectional shapesthe diameter, which interchangeably is referred to as width, is intendedto refer to the effective diameter.

While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiments,it is to be understood that the invention is not to be limited to thedisclosed embodiments, on the contrary, it is intended to cover variousmodifications and equivalent arrangements within the appended claims.

The invention claimed is:
 1. A method for forming nanowires comprising:providing metal catalytic seed particles suspended in a gas, providingGroup III and Group V gaseous precursors, that comprise constituents ofthe nanowires to be formed, dissolving the Group III material into themetal catalytic seed particles at a temperature of 380° C. to 700° C.;making at least one seed crystal at the surface of the at least onecatalytic seed particle, and growing epitaxially at least one nanowirecrystal from the at least one formed seed crystal in a gas-phasesynthesis including the gaseous precursors while the catalytic seedparticles are suspended in the gas and the constituents of the nanowiresto be formed are supersaturated in the at least one catalyst seedparticle, wherein the at least one nanowire crystal is a III-Vsemiconductor crystal which comprises gallium and arsenic.
 2. The methodof claim 1, wherein the nanowires are formed in a continuous process. 3.The method of claim 1, wherein the nanowires formed are carried by thegas.
 4. The method of claim 1, wherein the growth conditions duringgrowth of each nanowire are varied by controlling one or more ofparameters associated with: precursor composition, precursor molar flow,carrier gas flow, temperature, pressure or dopants, such that a nanowiresegment is axially grown on a previously formed nanowire portion in alongitudinal direction thereof, or a shell is radially grown on thepreviously formed nanowire portion in a radial direction thereof, ormaterial is added as a combination of axial and radial growth.
 5. Themethod of claim 4, wherein the growth conditions are varied to obtainheterostructures with respect to composition, doping, conductivity typewithin each nanowire.
 6. The method of claim 4, wherein the growthconditions are varied over time by controlling one or more of parametersassociated with: precursor composition, precursor molar flow, carriergas flow, temperature, pressure or dopants, or the size distribution ofthe catalytic seed particles is varied, such that nanowires withdifferent properties are formed.
 7. The method of claim 1, wherein thecatalytic seed particles are provided as an aerosol that is mixed withthe gaseous precursors.
 8. The method of claim 1, wherein the catalyticseed particles are provided by formation from gaseous reactants thatcomprises at least one of the constituents of the catalytic particles.9. The method claim 1, wherein the gas containing the catalytic seedparticles flows sequentially through one or more reaction zones, eachreaction zone contributes to the nanowire growth by adding material tothe nanowire, and the nanowires grown after passage through eachreaction zone are carried by the gas.
 10. The method of claim 1, whereinthe catalytic seed particles are charged.
 11. The method of claim 1,further comprising in-situ analysis of the nanowires formed.
 12. Themethod of claim 11, further comprising controlling the nanowire growthby feedback from in-situ analysis parameters without interrupting thenanowire forming process.
 13. The method of claim 11, wherein thein-situ analysis comprises illumination of the nanowires formed anddetection of luminescence from the nanowires to determine opticalproperties of the nanowires.
 14. The method of claim 1, furthercomprising depositing and/or aligning the nanowires on a substrate. 15.The method of claim 1, wherein the nanowires comprise a first portionand a second portion, wherein the first portion has a first compositionor a first conductivity type and the second portion has a secondcomposition or a second conductivity type, wherein the first compositionor the first conductivity type is different from the second compositionor the second conductivity type.
 16. The method of claim 1, wherein: themetal catalytic seed particles comprise gold catalytic seed particles;the Group III and Group V gaseous precursors comprise gallium andarsenic containing precursors; and dissolving the Group III materialinto the metal catalytic seed particles at a temperature of 380° C. to700° C. comprises dissolving gallium into gold catalytic seed particlesto form Au—Ga seed particles.
 17. The method of claim 16, wherein themetal catalytic seed particles comprise molten catalytic seed particles.18. The method of claim 16, wherein a gaseous precursor is mixed withthe gold catalytic seed particles prior to initiation of nanowiregrowth.
 19. The method of claim 1, wherein the at least one nanowirecrystal comprises a gallium arsenide nanowire crystal.
 20. The method ofclaim 1, wherein the nanowires have a formula GaAs orIn_(x)Ga_(1-x)As_(y)P_(1-y).
 21. The method of claim 1, wherein thenanowires comprise a pn or pin junction in which a p or n doped nanowiresegment is grown axially on another one of the p or n doped segment. 22.The method of claim 1, wherein a ratio of the Group V precursor to theGroup III precursor is between 0.2 and
 5. 23. The method of claim 1,wherein growing epitaxially at least one nanowire crystal occurs at areactor pressure of between 50 and 1100 mbar.